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Thread: subfloor in basement which is correct

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  1. #1
    DIY Member ctkeebler's Avatar
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    Default subfloor in basement which is correct

    Hello

    I am remodeling/refinishing my basement in Connecticut. I have a question about putting down a subfloor. What would be the recommended or preferred way?

    (1) Put 1 inch XPS foam directly on the concrete floor then 3/4 or 5/8 plywood on top and tapcon both the wood and foam to the concrete floor.

    (2) Put down 6mm vapor barrier and use 1x4 pt sleepers with 3/4 or 5/8 plywood on top and use either 16 or 24 on center spacing for the sleepers. With 24 inch spacing can put 1/2 inch foam between the sleepers but there will be a gap between the foam and the plywood unlike option #1. where the plywood is directly ontop of the foam.

    (3) Put down a plastic dimpled membrane and put the plywood directly on top of the dimpled membrane.

    I ask becasue Im replacing a dricore floor that failed due to water. The water must have wicked through the seams of the dricore.

    Im also adding a sump pump but Ill post those questions in the proper forum. Thanks
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    In the trades Dana's Avatar
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    Poly sheeting is far more vapor retardent than 1" of XPS (~0.05 perms vs. ~2.0perms), but if it's a bulk-water/high water table condition forcing liquid up through cracks the slab neither is going to save you completely. A hybrid of 1 & 2, with poly next to the concrete followed by 1" of XPS will be about as good as can be done. Minor amounts of water that wicks/seeps up the fastener holes can still easily dry through the XPS without creating mold conditions at the subfloor, and the high vapor retardency of the poly keeps the majority of the ground water where it belongs ( in the ground.).

    By putting all of the wood above the XPS (not sleepers on the slab) the wood stays warmer, and is less likely to retain or condense water on it. If you go with sleepers going directly on the slab they should be pressure-treated/ground-contact tolerant, since they'll likely be below the dew point of the interior air much of the year. Cool wood won't be able to dry readily, and will retain moisture.

    Separate sheets of poly should be lapped at least 8-12" and sealed with mastic at the edges, and extended up at least 6" on the foundation wall & mastic-sealed for best results. If you can't get at the foundation edges, use expanding foam to seal it best you can. The foam sealant is still semi vapor permeable in thin layers like that, but it's water proof and won't wick any ground moisture or condensation that might form on the foundation wall.

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    DIY Member ctkeebler's Avatar
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    Quote Originally Posted by Dana View Post
    Poly sheeting is far more vapor retardent than 1" of XPS (~0.05 perms vs. ~2.0perms), but if it's a bulk-water/high water table condition forcing liquid up through cracks the slab neither is going to save you completely. A hybrid of 1 & 2, with poly next to the concrete followed by 1" of XPS will be about as good as can be done. Minor amounts of water that wicks/seeps up the fastener holes can still easily dry through the XPS without creating mold conditions at the subfloor, and the high vapor retardency of the poly keeps the majority of the ground water where it belongs ( in the ground.).

    By putting all of the wood above the XPS (not sleepers on the slab) the wood stays warmer, and is less likely to retain or condense water on it. If you go with sleepers going directly on the slab they should be pressure-treated/ground-contact tolerant, since they'll likely be below the dew point of the interior air much of the year. Cool wood won't be able to dry readily, and will retain moisture.

    Separate sheets of poly should be lapped at least 8-12" and sealed with mastic at the edges, and extended up at least 6" on the foundation wall & mastic-sealed for best results. If you can't get at the foundation edges, use expanding foam to seal it best you can. The foam sealant is still semi vapor permeable in thin layers like that, but it's water proof and won't wick any ground moisture or condensation that might form on the foundation wall.

    Dane Thanks,

    What are your thoughts on the use of a dimpled plastic membrane like the Delta FL or planton, then either 1/2 inch or 1 inch foam and then plywood on top of that. OR just the dimpled plastic membrance and plywood on top of that?

    Is OSB or Plywood a better option?

    Thanks Ill look forward to your responses

    Im not sure how strong the 1/2 inch foam is. I know the 2inch foam I used on the walls is very strong to stand on.

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    Moderator & Master Plumber hj's Avatar
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    OSB swells in the presence of water, so I would NOT use it for a subfloor. You are putting a sheet of plywood over the foam, so your weight will be well distributed over the surface, so it is not as if you were standing directly on the foam.

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    In the trades Dana's Avatar
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    I'm not sure what the advantage of the dimpled plastic would be in this application- the bottom of the subfloor will be the same temp & humidity with or without the air gap. With foam in there it's HUGE advantage, since it keeps the temp of the wood higher, which keeps it's vapor pressure up- it dries.

    With foam directly below the subfloor there's no structural load to spaeak of- the compressive strength of the foam is adding a LOT of structure. At 3/4" OSB is rated as subfloor under wood flooring even with 18" o.c. joist spacing. You can get away with 7/16" stuff under wood flooring if its a concrete/poly/XPS/OSB sandwich, no air gaps to flex. But if it's going to be under carpet or tile go with at least 5/8". XPS has a very significant compressive strength (~15psi / 10%), and is used under heavy concrete all the time- I wouldn't sweat it a bit under residential flloor dynamic loading with 5/8" of OSB on top.

    As for swelling concerns, thats only an issue if you anticipate it flooding, and in that unhappy eventuality you'd be looking at ripping it up whether it was plywood or OSB. With a class-I vapor retarder on the floor it won't be picking up ground moisture, and with R5 between it and the cool earth both the top & bottom will be pretty much at room temp & humidity. I wouldn't put either on a basement floor in CT with no insulation, vapor retarder only, since it's ~R 0.75 insulation value will keep the bottom side cold enough to retain moisture- under a thick carpet even the top side could be cold enough to grow mold. But with a total R-stackup of R6-8 (depending on finish floor materials), all of the wood will be much closer to room temp than ground temp, and can't stay wet enough to rot, mold or swell. If you keep the basement below 60% relative humidity (the high end of the ASHRAE recommended for health- keep it under 50% if anybody is allergic to dust mites) it'll be fine. In CT this usually means running a dehumidifier/AC and keeping outdoor air from entering during the summer, since outdoor dewpoints tend to be high, and outdoor-air ventilation into a cool basement raises, not lowers the relative humidity. At 70% RH & up the mold hazard skyrockets.

  6. #6
    DIY Member ctkeebler's Avatar
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    Quote Originally Posted by Dana View Post
    I'm not sure what the advantage of the dimpled plastic would be in this application- the bottom of the subfloor will be the same temp & humidity with or without the air gap. With foam in there it's HUGE advantage, since it keeps the temp of the wood higher, which keeps it's vapor pressure up- it dries.

    With foam directly below the subfloor there's no structural load to spaeak of- the compressive strength of the foam is adding a LOT of structure. At 3/4" OSB is rated as subfloor under wood flooring even with 18" o.c. joist spacing. You can get away with 7/16" stuff under wood flooring if its a concrete/poly/XPS/OSB sandwich, no air gaps to flex. But if it's going to be under carpet or tile go with at least 5/8". XPS has a very significant compressive strength (~15psi / 10%), and is used under heavy concrete all the time- I wouldn't sweat it a bit under residential flloor dynamic loading with 5/8" of OSB on top.

    As for swelling concerns, thats only an issue if you anticipate it flooding, and in that unhappy eventuality you'd be looking at ripping it up whether it was plywood or OSB. With a class-I vapor retarder on the floor it won't be picking up ground moisture, and with R5 between it and the cool earth both the top & bottom will be pretty much at room temp & humidity. I wouldn't put either on a basement floor in CT with no insulation, vapor retarder only, since it's ~R 0.75 insulation value will keep the bottom side cold enough to retain moisture- under a thick carpet even the top side could be cold enough to grow mold. But with a total R-stackup of R6-8 (depending on finish floor materials), all of the wood will be much closer to room temp than ground temp, and can't stay wet enough to rot, mold or swell. If you keep the basement below 60% relative humidity (the high end of the ASHRAE recommended for health- keep it under 50% if anybody is allergic to dust mites) it'll be fine. In CT this usually means running a dehumidifier/AC and keeping outdoor air from entering during the summer, since outdoor dewpoints tend to be high, and outdoor-air ventilation into a cool basement raises, not lowers the relative humidity. At 70% RH & up the mold hazard skyrockets.

    Thanks Dana, I guess the thought of the dimpled plastic was for air movement to keep it dry if there was any moisture or seepage. So your recommendation is a 6ml poly vapor barrier, then the XPS foam and 5/8 plywood/osb on top of that all tapcon to the concrete floor?

    One inch minimum on the XPS foam or can I get away with 1/2 inch? Well never mind looks like the ownes corning XPS pink board is 1 inch (R5) or 3/4 of an inch (3.8)

    So I guess it doesnt really matter the size of the foam as the 1/4 wont affect the height that much. Might as well spend the $2.00 more per board now and get the better R value. maybe Ill save money in the long run and the wood is that much higher (away) from the cold concrete floor.


    I'm also looking at the Zoeller M98 sump pump. Probably over kill, but want to make sure there is enough room to space if there is another huge rain storm.

  7. #7
    DIY Senior Member dlarrivee's Avatar
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    I can't lose another inch in my basement, I'm using Delta FL with 5/8" plywood.

    OSB should be avoided.

    1" of XPS would be great if you have lots of headroom already.

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